The displays on graphical computer terminals are generated by reading a "bitmap" (i.e., a storage array of "1s" and "0s" corresponding to the intensity pattern of the display screen) and using the bits to intensity-modulate the electron beam of the cathode ray tube. The display is maintained by re-reading the bitmap at the frame rate of the display screen. Changes in the display are accomplished by changing the bitmap. Bits can be erased to remove display segments, or new bit patterns can be ORed with the existing bit pattern to create an overlay in the bitmap.
It is well known to break the bitmap, and hence the display, into a plurality of regions for separate displays. Each separate display is called a "window" and the prior art has the ability to display multiple windows simultaneously, with several if not all windows overlapping, leaving one window fully visible and the others partially or wholly obscured. Windows are overlapping rectangles each of which can be considered an operating environment, much like sheets of paper on a desk. One limitation of the prior art is that only the window at the front, which is totally unobscured, is active or continuously operating. The user is therefore limited to interacting with only the one active window and is prevented from operating on any of the obscured areas. The windows are typically not independent; each is supported by a separate subroutine in a single large program.
While the user interacts with the active window, all the remaining window programs are executing, but the results are not visible on the screen. If the user wants to view the progress of a particular program, it is necessary to poll the inactive windows periodically. This polling requires interrupting the users current work on the one active window in order to call up the desired window. At this point the bitmap for the obscured window would have to be updated in order to be displayed on the cathode ray tube (CRT) in the current state. One such system is the Xerox Smalltalk-80 system described in Vol. 6, No. 8, of the publication, BYTE, McGraw-Hill, August 1981.